Microsoft says that users of the free Windows Live Mail 2012 on Windows 7 who have an Outlook.com account won't be able to send or receive emails from that app after June 30.
Microsoft explained what is going on in a blog post:
To provide you an email experience with enhanced performance, security and reliability, the new Outlook.com has been built upon our latest Office 365 infrastructure. Because the Windows Live Mail 2012 email application doesn't support the protocols used by this new infrastructure, you will not be able to send or receive Outlook.com email from Windows Live Mail 2012 after your account is upgraded. This means the time has come for you to upgrade to a new email application.
Microsoft says that Windows 7 users of Windows Live Mail 2012 will be able to use the included Mail app to keep getting their Outlook.com messages if they upgrade their OS to Windows 8.1 or Windows 10. If they don't want to upgrade just yet, they can still access their Outlook.com emails via a web browser in Windows 7. Microsoft will also provide those users a free one-year subscription to Office 365 Personal. It includes Outlook 2016, which can send and receive Outlook.com messages.